1. Field of the Invention
The invention relates to an image sensor. More particularly, the invention concerns a structure for an image sensor integrated with a semiconductor device having enhanced blue light transmittance.
2. Description of the Related Art
Charge coupled devices (CCD) are currently used as image sensors. CCD technology has been developed over many years and is now mature and stable. Complementary metal oxide semiconductor (CMOS) image sensor technology is newer than CCD technology, and although this newer technology is lower in resolution and quality than a CCD, a CMOS image sensor still has other advantages such as lower fabrication costs due to the use of the CMOS fabrication process.
Although the technology for the CMOS image sensor is not stable and most of processes are still being researched, when compared with the CCD, the CMOS image sensor is more easily integrated with a wafer for purposes such as image processing. Accordingly, the integration of IC devices for a CMOS image sensor can therefore be greatly increased. Fabrication costs are thereby brought down, dimensions are reduced, and power consumption is decreased. All these advantages increase the value of the IC device. Therefore, it is predicted that the CMOS image sensor will take the place of the CCD and play a major role in the future.
However, with respect to a CMOS image sensor, transmittance of light for the semiconductor structure used in a semiconductor image sensor is an important factor seriously influences the quality of the image sensor. For example, it is imperative that the light transmittance is high enough. Only a high transmittance enables the light to arrive at the depletion region with a sufficiently high electric field in the semiconductor substrate. Upon arrival, the transmitted light induces electron-hole pairs due to excitation of photo-energy and thereby produces current in the intrinsic depletion region when light with varied wavelengths penetrates the passivation layer protecting the semiconductor structure.
Referring to FIG. 1, a metal layer 102 is formed over a semiconductor substrate 100 having a CMOS sensor (not shown) to connect with the device on the semiconductor 100. A passivation layer 104 consisting of phosphosilicate glass (PSG) and silicon nitride (SiN.sub.x) with a thickness of about 5000 angstroms and about 7000 angstroms, respectively, is formed on the metal layer 102 to protect the underlying devices from being damaged. Due to the formation of devices on the semiconductor substrate 100, and especially to the presence of the metal layer 102, the surface of passivation layer 104 is extremely uneven. It is necessary to form a plain film 106 on the passivation 104 to planarize the passivation layer topography. The plain film 106 can be made from polyimide or acrylic resin, for example, to a thickness of about 16000 angstroms. Thereafter, a color filter 108 is formed on the plain film 106 and light reaches the semiconductor substrate 100 through the color filter 108. A plain film 110 is then formed on the color filter 108 to a thickness of about 10000 angstroms for protecting the color filter 108. Accordingly, the color filter 108 is protected from destruction and contamination by moisture when the etching process for exposing the bonding pad is performed.
Blue light transmittance by the silicon nitride in the passivation layer 104 is about 70% when the light passes through the blue color filter 108, since wavelength of the blue light, about 460 nanometers, is shorter, as shown in FIG. 2. The curve 200 in FIG. 2 represents the transmittance of the silicon nitride when the light passes through the blue color filter to penetrate the silicon nitride. Transmittance of the plain film 104 as the light passes through is about 95%. As a result, the total transmittance of these three films (plain films 104, 110 and silicon nitride layer) is merely approximately 63.2%. Because the blue light transmits the silicon nitride so poorly, the semiconductor substrate 100 receives insufficient light, which light is incapable of exciting enough electrons. This causes the color to change to yellow. In addition, since the plain film itself is made from polymeric material, polymer is easily produced and covers the wafer when etching the plain film 104 and passivation layer 104 to expose the bonding pad. The etching rate of the layers obviously decreases when the polymer covers the layers that need to be etched. Also, the plain films 106 and 110 are respectively almost 16000 and 10000 angstroms thick, and polymer suspended in the etching gas, such as oxygen, for example, cannot be entirely carried out of the etching chamber by the etching gas when the etching gas is evacuated. . For these reasons, the etching process is conducted with difficulty. Accordingly, the etching process for the wafer requires at least 6 minutes and preventive maintenance (PM) to keep the reaction chamber clean is required after etching 15 wafers. The etching time is too long, the interval between preventive maintenances (PMs) is too short and etchant for etching the plain film 106 is also expensive, so there is no potential for using this process to fabricate a semiconductor image sensor in line and produce product in quantity.